1. Field of the Invention
The invention relates to high impact strength polymer blends that contain two two-phase or multiphase polymers P1 and P2, with the hard (matrix) phases B1 and B2 of the polymers P1 and P2 differing in their chemical structure and being compatible with one another. These polymer blends may optionally contain another thermoplastic polymer B'3 and/or another two-phase or multiphase polymer P4, with the hard phase B4 of the polymer P4 being different from B1 and B2 and compatible with B1 and B2. The polymer blends of the invention can be used as injection molding or extrusion compositions for plastic parts subjected to high stresses, for example casings, shock absorbers, or load-bearing mechanisms.
2. Discussion of the Background
The problem of converting relatively brittle plastics into industrially usable products with improved impact strength is well known.
Methods are described in detail in the literature for impact strength modification of plastics, especially thermoplastics. Thus, for example, D. Neufray and K.-H. Ott report the impact strength modification of polybutylene terephthalate, polyamides, and polycarbonates with butadiene graft polymers (Angew. Makromol. Chem., 1981, 98:213-224). Crosslinked rubber particles are anchored or coupled to the thermoplastic matrix, since otherwise no improvement of toughness is achieved even with high elastomer content. The coupling can be of physical or chemical nature, i.e., it is achieved by grafting the crosslinked rubber particles to polymers that are polymerizable by themselves into thermoplastic polymers. The number of entanglements that the grafted polymer molecules develop with the matrix polymer molecules and the thermodynamic compatibility between grafted polymer molecules and matrix polymer molecules are said to be crucial for the coupling.
In general, therefore, grafted branch polymers that are identical with the matrix polymers are chosen. For example, elastomer particles consisting of crosslinked polybutadiene or polybutyl acrylate that have PMMA grafted branches are used for the impact strength modification of polymethyl methacrylate (PMMA). See, for example, EP 0 113 924 (equivalent to U.S. Pat. No. 4,513,118), DE-OS 33 29 765 (equivalent to U.S. Pat. No. 4,521,567) and EP 0 080 072.
A distinct increase of impact strength, particularly of notched impact strength, can be produced for brittle plastics when the grafted branch polymer particles bound to the crosslinked elastic particles are different chemically from the matrix polymer, but the grafted branch polymers are thermodynamically compatible with the matrix polymers, in other words they form polymer alloys. Such impact strength modifying agents and their blends with polyvinyl chloride, polystyrene, poly-.alpha.-methylstyrene, chlorinated rubber, polycarbonate, and copolymers of methyl methacrylate and .alpha.-methylstyrene are described in DE-OS 37 43 199 (equivalent to U.S. Pat. No. 4,906,609).
Impact strength modifying agents that have grafted branches whose compatibility is specifically designed for polycarbonate are described in DE-OS 38 03 405 (equivalent to U.S. Pat. No. 4,997,883).
Polymer blends of two elastomer-modified polymers that have the same grafted branch polymers and matrix polymers but different toughness phases are described in EP-A 0 370 345 and EP-A 0 370 346. If the toughening phases have definite particle sizes, combinations with good toughening at low temperatures and good thermoplastic processability can be produced. EP-A 0 278 347 includes thermoplastic molding compositions from rubber-modified vinylaromatic polymers and polyalkylene ethers that have grafted branches of a copolymer consisting of alkyl acrylates, cycloalkyl (meth)acrylates, and optionally styrene, and show antistatic behavior.
EP-A 0 367 052 relates to rubber-modified molding compositions containing thermoplastic polyesters and a mixture of two grafted rubbers consisting of diene rubber with grafted branch copolymers of styrene, acrylonitrile, and methyl methacrylate, and acrylate rubber with the same grafted branch copolymers, which show good toughness at low temperatures.
Although a number of patents and other literature exist on the subject of "impact strength modification of thermoplastics", there is a great industrial need for impact strength modifying agents that distinctly increase the impact strength and notched impact strength of thermoplastics while retaining their other positive properties. Thus, as described in DE-OS 33 29 765 (equivalent to U.S. Pat. No. 4,521,567), the impact strength modification of polymethyl methacrylate (PMMA) is increased by a factor of 5 compared to unmodified PMMA by a 30 wt. % elastomer fraction, but at the same time the Vicat temperature is reduced by 20.degree. K., which is prohibitive for many applications, especially for injection molded parts subjected to severe thermal stresses.
The blends described in EP-A 370 345 and EP-A 370 346 of crosslinked silicone and acrylate rubbers grafted with styrene-acrylonitrile copolymers and styrene-acrylonitrile (SAN) copolymers show improved flow capabilities compared to SAN copolymers that are modified only with acrylate elastomers, but on the other hand they have no significantly increased notched impact strength.
EP-A 0 367 052 describes molding compositions of polyesters and a mixture of two grafted rubbers, an acrylate rubber and a butadiene grafted rubber, whose low-temperature notched impact strength rises continuously with an increase in the proportion of butadiene grafted rubber in the rubber blend. Of course, this is combined with a decrease of the modulus of elasticity.